The thermoplastic resin has the properties of softening when heated and hardening when cooled, and does not undergo a chemical reaction in the process, and the properties of softening when heated and hardening when cooled can be maintained through repeated heating and cooling processes. Among them, the characteristic temperature of the crystalline resin is the melting point, and the characteristic temperature of the non-crystalline resin is the glass transition temperature. The thermoplastic resin has the following characteristics: the thermoplastic resin is high-molecular-weight solid at room temperature, is a linear polymer or a polymer with a small amount of branched chains, has no intermolecular crosslinking, and attracts each other only by virtue of van der Waals force or hydrogen bonds. In the molding process, the resin is pressurized and heated to be softened and to flow, is not chemically crosslinked, can be shaped in the mold, and is then cooled and molded to get a product of a certain shape. In the repeated heating process, the molecular structure is basically not changed, but when the temperature is too high, and the time is too long, crosslinking, degradation or decomposition will occur.
The thermoplastic resin includes polyethylene (PE), polypropylene (PP), polybutylene (PB), polyester (polyethylene terephthalate (PET), polybutylene terephthalate (PBT)), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polystyrene (PS), polyamide (PA), polyoxymethylene (POM), polycarbonate (PC), polyphenylene oxide (PPO), polysulfone (PSF) and the like, and biodegradable resin such as polylactic acid (PLA), polybutylene succinate (PBS), polybutylene succinate-co-butylene terephthalate (PBST), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), and the like.
With the development of society, in the automotive, aircraft, logistics, packaging and other industries, specific requirements have made for lightweighting, which meets the requirements of circular and green economy. Thermoplastics and the resulting foamed materials are the most important lightweight materials that not only play an important role in advancing the lightweight process, but also provide higher design freedom degree and differentiation. Lightweighting does not mean reducing the original safety and use performance of the materials. As long as the design, material selection and manufacturing processes are reasonable, lightweighting can meet the safety, vibration and noise standards and durability requirements, to ensure the use performance. For example, in the field of automotive manufacturing, in addition to the body, the automotive parts and interior and exterior trimming parts make a crucial contribution to achieving lightweighting, energy saving, safety, comfort and other goals, and the bumper cores, ceiling, interior trims and shading panels of the automotive, and even the automotive seats can be replaced with foamed thermoplastics.
Due to the characteristics of lightweight and good mechanical properties, and capability of being prepared into products of specific shapes through molding, expanded polypropylene (EPP) beads are a widely used polymer foamed material, and is the focus of national industrial and academic attention in the aspects of development and industrial production. The foam molding of EPP beads obtained by molding the EPP beads has excellent performance of chemical resistance, high toughness, high heat resistance, good compression resilience and the like as compared with foam moldings of expanded polystyrene-series resin beads. But the currently industrial EPP beads have the shortcomings of high molding temperature, poor flame retardancy and resistance to static electricity, poor low-temperature impact resistance and the like.
At first, the molding energy consumption is high. When the EPP beads undergo insert molding, in order to allow the expanded beads to melt in contact with each other at the same time as the expanded beads are secondarily foamed, it is necessary to use water vapor having higher saturated vapor pressure to heat. Therefore, it is necessary to use a high-pressure-resistance metal mold and a high-stamping special molding machine, which leads to an increase in energy cost, so it is very important to develop an EPP bead molding process with lower vapor pressure and lower temperature.
Secondly, the EPP beads are flammable. Polypropylene is a flammable substance, and emits more heat while burning, accompanied by droplets, and thus the flame is extremely easy to spread. In addition, the EPP beads have a cell structure, and thus have worse flame-retardant property. At present, most of the EPP beads cannot achieve flame retardant function, thus limiting the applications in the field with high flame-retardant requirement. Currently, flame-retardant PP is mainly produced by using a flame retardant compounded by halogen-containing organic compounds with antimony trioxide in the domestic market. The plastic products of the halogen-containing flame retardant will produce toxic and corrosive gases and a lot of smoke in the combustion, and great harm to the environment is caused. In recent years, many environmental assessment reports indicated that the halogen flame-retardant materials released benzofuran, dioxin and other highly toxic carcinogens in the processing, combustion and recycling processes, and generated serious harm to the environment and human health. In February of 2003, the EU firstly announced the ROHS (Restriction of Hazardous Substances) for limiting use of halogens, and Germany, the United States, Japan and China also published the relevant environmental laws and regulations. To guarantee that products and production lines meet the requirements of the existing and future environmental regulations, the global producers, suppliers and customers of electrical and electronic equipment make the safest requirements-“zero halogen” in the internal supply chain.
At present, the typically used polypropylene halogen-free flame retardants include hydroxide, phosphorus series, nitrogen series and a complex thereof. The typical hydroxide flame retardants are magnesium hydroxide and aluminum hydroxide, and polypropylene can reach the UL94 V0 flame-retardant level required by an insulating sheet when the addition amount is more than or equal to 60 wt %, but this leads to the difficulty in flame-retardant polypropylene processing. The typical phosphorus-series flame retardants are red phosphorus and organic phosphates, and the addition amount is lower than that of hydroxides, but the insulation level of polypropylene plates is reduced due to large water absorption rate and high permeability. The typical nitrogen-series flame retardants are melamines and triazines, but the product cannot achieve a high flame-retardant level when the thickness of the foam molding or plate is in the range of 0.125-0.75 mm. Therefore, the development of a low-smoke halogen-free environment-friendly flame-retardant PP composite material has very important practical significance.
Thirdly, the EPP beads have poor antistatic properties. When the molded EPP beads are used as relevant electronic material packages and liquid crystal panel turnover boxes, high antistatic performance is required. The common PP foamed material is poor in antistatic performance, and easily produces static charges in the friction with or stripping from the outside, and the charges are not easy to leak out and constantly accumulated on the surface. After the polypropylene surface is charged, polypropylene adsorbs the dust and dirt in the air if no effective surface treatment or antistatic treatment is carried out. When the human body is exposed to the electrostatic polypropylene, the human body feels electric shock, and static electricity can also cause the malfunction of electronic equipment, more seriously, the accumulation of static electricity will cause electrostatic attraction (or repulsion), electric shock or spark discharge phenomenon, and this will lead to a huge disaster in the flammable and explosive material environment. In order to avoid the influence of static electricity, the antistatic modification must be carried out on polypropylene to adapt to certain special occasions.
The addition of conductive functional bodies (such as conductive carbon black) or antistatic agents to a polymer matrix is one of main methods for preparing polymer-based antistatic composite materials. However, in general, the filling amount of the conductive filler or the addition amount of the antistatic agent required to form a conductive network is relatively large, resulting in a significant decrease in the mechanical properties and the like of the polymer, and the production cost and the process difficulty of the material are improved. Therefore, reducing the amount of the conductive filler is an important part of the development and application of antistatic composite materials. Chinese Patent Application 200510004023.0 describes the preparation of an antistatic polyolefin resin foam molding using a polymer antistatic agent, the obtained foam molding has intrinsic surface resistivity of 108-1013Ω, and the used polymer antistatic agent mainly comprises a polyether-polypropylene block copolymer, a mixture of polyether ester amide and polyamide, and the like, but the antistatic agent is added in an amount of 4-6 wt %, is a short-acting antistatic agent, and is valid for only 30 days. Chinese Patent Application 200710192215.8 describes a preparation method of antistatic and anti-conductive polypropylene, the volume resistivity of the obtained polypropylene pellets is adjustable in the range of 1010-1011 Ω·cm, and the addition amount of carbon black is 25-35 wt %; the carbon black has low apparent density, larger addition amount, and difficulty in blending with polypropylene base resin, thereby increasing the complexity of the process and product cost.
Most importantly, after the polypropylene beads are added with a flame retardant and a long-acting antistatic agent, the cell structure of the EPP beads and the mechanical properties of the foam molding are significantly affected, and the quality of the molded products obtained through subsequent molding is difficult to guarantee, thus limiting its application areas. When the flame retardant and the antistatic agent are both added, the decrease in the flame retardancy of the flame retardant or antistatic properties of the antistatic agent is often caused.
Fourthly, polypropylene, especially propylene homopolymer, is poor in low-temperature impact resistance. The impact polypropylene obtained by adding a rubber dispersion phase has excellent high- and low-temperature impact strength, high tensile strength, flexural modulus and rigidity and high heat resistance temperature, and is widely used in many fields such as molded or extruded automotive parts, household appliances, containers and household items. The expanded beads prepared from the impact polypropylene also have good resistance to low temperature, and especially have a wide prospect in cold chain transport packages, sports equipment, building insulation, and aerospace. The conventional general-grade impact polypropylene has the problems of combined cell breakage, low molding capability and the like due to low melt strength when used for preparing the expanded beads.
A common method for increasing the melt strength of polypropylene is to reduce the melt index, i.e., to increase the molecular weight of polypropylene, but this may lead to difficulty in melting and extruding the material. Another method is to widen the molecular weight distribution, for example, U.S. Pat. Nos. 7,365,136 and 6,875,826 describe a method for preparing homopolypropylene and random copolypropylene with wide molecular weight distribution and high melt strength, wherein alkoxysilane (such as dicyclopentyldimethoxysilane) is selected as an external electron donor, and the effect of increasing the melt strength of polypropylene is achieved by adjusting the hydrogen concentration in a plurality of serial reactors to regulate the size and distribution of the molecular weight. WO 9426794 discloses a method for preparing homopolypropylene and random polypropylene with high melt strength in a plurality of serial reactors, wherein polypropylene with high melt strength and wide molecular weight distribution or bimodal distribution is prepared by adjusting the hydrogen concentration in different reactors, and the property of the catalyst is not adjusted in each reactor, so the preparation process requires a lot of hydrogen. CN 102134290 and CN 102134291 disclose a method for preparation of homopolypropylene with wide molecular weight distribution and high melt strength, wherein a plurality of serial reactors are adopted, and homopolypropylene or random copolypropylene with wide molecular weight distribution and high melt strength is prepared by controlling the type and proportion of the external electron donor components at different reaction stages and then controlling the amount of the molecular weight regulator hydrogen. Chinese Patent Application 201210422726.5 also reports a preparation method of homopolypropylene or random copolypropylene with wide molecular weight distribution and high melt strength, wherein the isotactic index and hydrogen response of the catalyst between the different reactors are regulated by reasonable matching of two different types of external electron donors of silanes and diethers.